RF Front-End with On-Chip Transmitter/Receiver Isolation Using A Gyrator

ABSTRACT

An RF front-end with on-chip transmitter/receiver isolation using a gyrator is presented herein. The RF front end is configured to support full-duplex communication and includes a gyrator and a transformer. The gyrator includes two transistors that are configured to isolate the input of a low-noise amplifier (LNA) from the output of a power amplifier (PA). The gyrator is further configured to isolate the output of the PA from the input of the LNA. The gyrator is at least partially or fully capable of being integrated on silicon-based substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. Provisional PatentApplication No. 61/291,152, filed Dec. 30, 2009, entitled “RF Front-EndWith On-Chip Transmitter/Receiver Isolation Using a Gyrator,” which isincorporated herein be reference in its entirety.

FIELD OF THE INVENTION

This application relates generally to wireless communication systems,and more particularly to full-duplex radio frequency (RF) transceiversthat operate in such systems.

BACKGROUND

A duplex communication system includes two interconnected transceiversthat communicate with each other in both directions. There are multipletypes of duplex communication systems; including, half-duplexcommunication systems and full-duplex communication systems. In ahalf-duplex communication system, the two interconnected transceiverscommunicate with each other in both directions. However, thecommunication in a half-duplex system is limited to one direction at atime; that is, only one of the two interconnected transceivers transmitsat any given point in time, while the other receives. A full-duplexcommunication system, on the other hand, does not have such alimitation. Rather, in a full-duplex communication system, the twointerconnected transceivers can communicate with each othersimultaneously in both directions.

Wireless and/or mobile communication systems are often full-duplex asspecified by the standard(s) that they employ. For example, a commonfull duplex mobile communication standard includes Universal MobileTelecommunications System (UMTS). In these full-duplex communicationsystems, the transmitter typically uses one carrier frequency in a givenfrequency band (e.g., 900 MHz, 1800 MHz, 1900 MHz, 2100 MHz, etc.) andthe receiver uses a different carrier frequency in the same frequencyband. This scheme, where the transmitter and receiver operate overdifferent frequencies, is referred to as frequency division duplexing(FDD).

Despite using different frequencies, the signal strength of thetransmitted signal is often significantly greater than that of thereceived signal (e.g., by as much as 130 dB) at the transceiver. Assuch, the receiver is susceptible to interference from the transmittedsignal. In order to limit the interference, conventional transceiversinclude a duplexer, which utilizes frequency selectivity to provide50-60 dB of isolation between the transmitter and the receiver. However,to provide for today's high frequency communication standards, duplexersshould be built with high quality factor (Q-factor) and low lossmaterials, which currently cannot be done using silicon-basedtechnology. As such, duplexers are fabricated using special materialsand processes (e.g., ceramic, surface acoustic wave (SAW), film bulkacoustic wave (FBAR), etc.) that cannot be integrated with a transceiveron a silicon-based IC.

More recent implementations of full-duplex wireless transceivers operateover multiple frequency bands (e.g., there are 14 frequency bands forFDD-UMTS), which require a separate duplexer for each band in order tomeet the isolation requirement. As each duplexer is off-chip (i.e., notintegrated with the transceiver on the silicon based IC), the monetarycost and size for multi-band transceivers can become substantial.

Therefore, a need exists for a duplexer functional circuit that can befabricated using silicon-based technology such that it can beimplemented on the same integrated circuit as the transceiver.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 illustrates a block diagram of an RF front-end that providesisolation by frequency selection.

FIG. 2 illustrates a block diagram of an RF front-end that providesisolation using a gyrator, according to embodiments of the presentinvention.

FIG. 3 illustrates a schematic diagram of a gyrator, according toembodiments of the present invention.

FIG. 4 illustrates a behavioral model of the RF front-end illustrated inFIGS. 2 and 3, where superposition is used to analyze the RF front-endduring transmission of a signal, according to embodiments of the presentinvention.

FIG. 5 illustrates a behavioral model of the RF front-end illustrated inFIGS. 2 and 3, where superposition is used to analyze the RF front-endduring reception of a signal, according to embodiments of the presentinvention.

The present invention will be described with reference to theaccompanying drawings. The drawing in which an element first appears istypically indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the invention. However, itwill be apparent to those skilled in the art that the invention,including structures, systems, and methods, may be practiced withoutthese specific details. The description and representation herein arethe common means used by those experienced or skilled in the art to mosteffectively convey the substance of their work to others skilled in theart. In other instances, well-known methods, procedures, components, andcircuitry have not been described in detail to avoid unnecessarilyobscuring aspects of the invention.

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to effect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

1. ISOLATION BY FREQUENCY SELECTION

FIG. 1 illustrates a block diagram of an RF front-end 100 configured toprovide full-duplex communication. RF front-end 100 includes an antenna105, a duplexer 110, a low-noise amplifier (LNA) 115, a power amplifier(PA) 120, and an integrated-circuit (IC) 125. RF front-end 100 can beused within a cellular telephone, a laptop computer, a wireless localarea network (WLAN) station, and/or any other device that transmits andreceives RF signals.

In operation, RF front-end 100 transmits and receives RF signals overnon-overlapping portions of a particular frequency band (e.g., one ofthe 14 bands specified by FDD-UMTS, including the 900 MHz, 1800 MHz, and2100 MHz bands). By transmitting and receiving signals overnon-overlapping portions of a particular frequency band, the two signalsdo not interfere with each other and full-duplex communication can beachieved. For example, as illustrated in FIG. 1, both inbound andoutbound signals are simultaneously coupled between antenna 105 andduplexer 110 over a common signal path 130. In such an arrangement,duplexer 110 is used to couple common signal path 130 to both the inputof LNA 115 and to the output of PA 120. Duplexer 110 provides thenecessary coupling, while preventing strong outbound signals, producedby PA 120, from being coupled to the input of LNA 115.

As illustrated in FIG. 1, duplexer 110 is a three-port device having anantenna port 135, a transmit port 140, and a receive port 145. Antennaport 135 is coupled to transmit port 140 through a transmit band-passfilter, included in duplexer 110, and to receive port 145 through areceive band-pass filter, further included in duplexer 110. The passband of the transmit filter is centered within the frequency range ofthe outbound signals, which are received at node 150 from a transmitter(not shown). The pass band of the receive filter is centered within thefrequency range of the inbound signals, which are passed to a receiver(not shown) at node 155. The transmit and receive band-pass filters areconfigured such that their respective stop bands overlap with eachothers pass bands. In this way, the band-pass filters isolate the inputof LNA 115 from the strong outbound signals produced by PA 120. Intypical implementations, duplexer 110 must attenuate the strong outboundsignals by about 50-60 dB to prevent the outbound signals fromsaturating LNA 115.

Today's high frequency communication standards (e.g., FDD-UMTS) requirethat conventional frequency selective duplexers, such as duplexer 110,be built with very high Q-factor and low loss materials, which currentlycannot be done using silicon-based technology. As such, conventionalduplexers are fabricated using special materials and processes (e.g.,ceramic, surface acoustic wave (SAW), film bulk acoustic wave (FBAR),etc.) that cannot be integrated with a transceiver on a silicon-basedIC. In an embodiment, IC 125 is implemented using silicon-basedtechnology and includes at least portions of LNA 115, the transmitter(not shown) coupled at node 150, and the receiver (not shown) coupled atnode 155. Because conventional duplexer 110 typically cannot beintegrated on IC 125, due to the limitations of silicon-basedtechnology, duplexer 110 is provided for off-chip, thereby increasingthe size and cost of the radio transceiver.

In addition, more recent implementations of full-duplex radiotransceivers operate over multiple frequency bands (e.g., there are 14frequency bands for FDD-UMTS), which require a separate conventionalduplexer 110 for each band. In these multi-band transceivers, eachduplexer is off-chip, significantly increasing the size and cost of theradio transceiver.

Therefore, a need exists for a duplexer functional circuit that can befabricated using silicon-based technology such that it can beimplemented on the same integrated circuit as the radio transceiver.

2. ISOLATION BY GYRATOR 2.1 Overview

FIG. 2 illustrates a block diagram of an RF front-end 200 configured toprovide full-duplex communication, according to embodiments of thepresent invention. Unlike RF front-end 100, illustrated in FIG. 1, whichprovides isolation using frequency selection, RF front-end 200 providesisolation using an electrical approach. Specifically, as will beexplained further below, RF front-end 200 provides isolation using agyrator-based duplexer.

RF front-end 200 includes an antenna 205 (e.g., a signal transducer), anIC 210, a gyrator 215, an LNA 220, a PA 225, and a transformer thatincludes a primary winding 230 and a secondary winding 235. In anembodiment, IC 210 is implemented using a silicon-based technology andincludes at least portions of LNA 220, transformer windings 230 and 235,gyrator 215, a transmitter (not shown) coupled at node 250, and areceiver (not shown) coupled at node 255. RF front-end 200 can be usedwithin a cellular telephone, a laptop computer, a wireless local areanetwork (WLAN) station, and/or any other device that transmits andreceives RF signals.

In operation, RF front-end 200 transmits and receives RF signals overoverlapping or non-overlapping portions of at least one particularfrequency band (e.g., one of the 14 bands specified by FDD-UMTS,including the 900 MHz, 1800 MHz, and 2100 MHz bands).

During reception, antenna 205 receives electromagnetic waves andconverts the electromagnetic waves into a modulated, electrical currentthat flows through winding 230. The varying current in winding 230induces a voltage across winding 235 that is sensed at the input of LNA220 and provided to a receiver at output node 255 of LNA 220.

During transmission, PA 225 receives a modulated, outbound signal atinput node 250, and produces an amplified version of the modulated,outbound signal. Specifically, PA 225 provides a modulated voltage atits output that leads to a current flowing through winding 235, which inturn induces a voltage in winding 230. Because antenna 205 represents aload coupled to winding 230, current will flow from winding 230 toantenna 205, where it will be converted to an electromagnetic wave andtransmitted.

Gyrator 215 is configured to prevent strong, outbound signals producedby PA 225 from saturating the input of LNA 220. Gyrator 215 is afour-terminal, two-port network which can be defined by the followingequations:

$\begin{matrix}{V_{X} = {I_{Y} \cdot \frac{1}{- g_{m\; 2}}}} & (1) \\{V_{Y} = {I_{X} \cdot \frac{1}{g_{m\; 1}}}} & (2)\end{matrix}$

where I_(X) is the current into and V_(X) is the voltage across the twoterminals constituting a first port 240 of gyrator 215, and I_(Y) is thecurrent into and V_(Y) is the voltage across the two terminalsconstituting a second port 245 of gyrator 215. Transfertransconductances g_(m1) and g_(m2) determine the gyration constant ofgyrator 215 and in one embodiment are substantially equal. Gyrator 215receives its name from the fact that it “gyrates” an input voltage intoan output current and vice versa as can be seen from equations (1) and(2) above.

To prevent the strong, outbound signals produced by PA 225 fromsaturating the input of LNA 220, gyrator 215 is configured to maintainthe component of the voltage V_(X), contributed by PA 225, at 0 volts.In other words, assuming no signal is being received, the voltage V_(X)is maintained by gyrator 215 to be substantially 0 volts duringtransmission, thereby isolating the output of PA 225 from the input ofLNA 220. The exact mechanism of isolation, provided by gyrator 215, willbe described further below in regard to FIGS. 4 and 5.

FIG. 3 illustrates a schematic diagram of gyrator 215, according toembodiments of the present invention. It should be noted that theschematic diagram of gyrator 215 illustrated in FIG. 3 is forillustration purposes only and gyrator 215 is in no way limited thereto.

Gyrator 215 includes two active elements: transistors M1 and M2 thateach have a source, drain, and gate node. The source and drain nodes oftransistor M1 are respectively coupled to the terminals of first port240 of gyrator 215, and the source and drain nodes of transistor M2 arerespectively coupled to the terminals of second port 245. In anembodiment, the respective sources of transistors M1 and M2 are furthercoupled to a ground potential, as illustrated in FIG. 3.

During operation, transistors M1 and M2 operate in saturation andessentially form two, common-source amplifiers that receive a controlvoltage at their respective gates and provide a drain-to-source currentproportional thereto. Specifically, transistor M1 is coupled to voltageV_(Y) at its gate and provides a drain-to-source current I_(X) that isapproximately given by:

I _(X) =V _(Y) ·g _(m1)  (3)

where g_(m1) is the transconductance of transistor M1. Transistor M2 iscoupled to voltage V_(X) through an inverting amplifier 300 and providesa drain-to-source current I_(Y) that is approximately given by:

I _(Y) =V _(X)·(−g _(m2))  (4)

where g_(m2) is the transconductance of transistor M2. The inversion ofthe transconductance of transistor M2, as noted in equation (4), isprovided by inverting amplifier 300. In an embodiment, invertingamplifier 300 is implemented as a common-source amplifier with unitygain.

By reordering equations (3) and (4) and solving for V_(Y) and V_(X), itcan be shown that the circuit implementation of gyrator 215 satisfiesequations (1) and (2) noted above and thereby produces gyrator action.The exact mechanism of isolation, provided by gyrator 215 duringtransmission will now be described further below in regard to FIG. 4.

2.2 Transmission

The principle of superposition, as applied to circuits, generally refersto the fact that the net response at any given node in a circuit due totwo or more independent sources is the sum of the responses caused byeach of the independent sources individually. In RF front-end 200, thereare effectively two independent sources: antenna 205 and PA 225. Antenna205 effectively behaves as an independent voltage source duringreception of an inbound signal, and PA 225 effectively behaves as anindependent voltage source during transmission of an outbound signal.

In FIG. 4, the principle of superposition is used to analyze theresponse of RF front-end 200 during transmission of a signal(independent from the response caused by the reception of a signal) todescribe the method of isolation provided by gyrator 215. FIG. 4specifically illustrates an equivalent circuit model 400 for RFfront-end 200 that includes an independent voltage source (V_(PA)) 405,resistor (R_(PA)) 410, resistor (R_(ANT)) 415, resistor (R_(LNA)) 420,and dependent current sources 425 and 430, which are included in gyrator215. Independent voltage source 405 is representative of the voltageprovided by PA 215 during transmission. R_(PA) 410 represents the outputresistance of PA 215, R_(ANT) 415 represents the resistance of theantenna 205 as seen from the secondary winding 235 in FIG. 2, andR_(LNA) 420 represents the input resistance of LNA 220. Dependentcurrent source 425 represents transistor M1 and dependent current source430 represents transistor M2. Dependent currents sources 425 and 430 aredefined by equations (3) and (4) noted above and reproduced below:

I _(X) =V _(Y) ·g _(m1)  (3)

I _(Y) =V _(X)·(−g _(m2))  (4)

where g_(m1) is the transconductance of transistor M1 and g_(m2) is thetransconductance of transistor M2.

By design, resistors 410, 415, and 420 each have substantially equalresistances (typically 50 ohms) in order to maximize power transfer andminimize reflections. In addition, by design the reciprocal of thetransconductances of transistors M1 and M2 (i.e., and 1/g_(m1) and1/g_(m2)) are further substantially equal to the resistance of resistors410, 415, and 420.

Given these design assumptions, in order for isolation to exist betweenthe input of LNA 220 and the output of PA 225, the voltage V_(X) shouldbe maintained at 0 volts during transmission. If V_(X) is 0 volts duringtransmission, then dependent current source 430 behaves as an opencircuit and supplies 0 amps of current as determined by equation (4)above. Therefore, the entire power amplifier current I_(PA), asillustrated in FIG. 4, flows through R_(ANT) 415. In order for V_(X) tobe maintained at zero volts, the current I_(X) provided by dependentcurrent source 425 should balance (or be equal) to the current I_(PA)flowing through R_(ANT) 415. The current I_(PA) is given by:

$\begin{matrix}{I_{PA} = {\frac{V_{Y} - V_{X}}{R_{ANT}} = {\frac{V_{Y} - 0}{R_{ANT}} = \frac{V_{Y}}{R_{ANT}}}}} & (5)\end{matrix}$

To verify that the current I_(X) provided by dependent current source425 is equal to the current I_(PA), equation (5) can be set equal toequation (3) above. If the resulting equation holds true (or is valid)then V_(X) is 0 volts and isolation has in fact been achieved:

$\begin{matrix}{I_{PA} = {\frac{V_{Y}}{R_{ANT}} = {{V_{Y} \cdot g_{m\; 1}} = I_{X}}}} & (6)\end{matrix}$

Because g_(m1) by design is equal to 1/R_(ANT), the above equation holdstrue so that the input of LNA 225 is isolated from the output of PA 220during transmission.

2.3 Reception

In FIG. 5, the principle of superposition is further used to analyze theresponse of RF front-end 200 during reception of a signal (independentfrom the response caused by the transmission of a signal). Specifically,FIG. 5 further illustrates that gyrator 215 further isolates the outputof PA 225 from the input of LNA 220 during reception.

FIG. 5 illustrates an equivalent circuit model 500 for RF front-end 200that includes an independent voltage source (V_(ANT)) 535, resistor(R_(PA)) 510, resistor (R_(ANT)) 515, resistor (R_(LNA)) 520, anddependent current sources 525 and 530, which are included in gyrator215. Independent voltage source 535 is representative of the voltageprovided by antenna 205 during reception. R_(PA) 510 represents theoutput resistance of PA 215, R_(ANT) 515 represents the resistance ofthe antenna 205 as seen from the secondary winding 235 in FIG. 2, andR_(LNA) 520 represents the input resistance of LNA 220. Dependentcurrent source 525 represents transistor M1 and dependent current source530 represents transistor M2. Dependent current sources 525 and 530 aredefined by equations (3) and (4) noted above and reproduced below:

I _(X) =V _(Y) ·g _(m1)  (3)

I _(Y) =V _(X)·(−g _(m2))  (4)

where g_(m1) is the transconductance of transistor M1 and g_(m2) is thetransconductance of transistor M2.

By design, resistors 510, 515, and 520 each have substantially equalresistances (typically 50 ohms) in order to maximize power transfer andminimize reflections. In addition, by design the reciprocal of thetransconductances of transistors M1 and M2 (i.e., 1/g_(m1) and 1/g_(m2))are further substantially equal to the resistance of resistors 510, 515,and 520.

Given these design assumptions, in order for isolation to exist betweenthe input of LNA 220 and the output of PA 225, the voltage V_(Y) shouldbe maintained at 0 volts during reception. If V_(Y) is 0 volts duringreception, then dependent current source 525 behaves as an open circuitand supplies 0 amps of current as determined by equation (3) above.Therefore, the entire antenna current I_(ANT), as illustrated in FIG. 4,flows through R_(LNA) 515. In order for V_(Y) to be maintained at zerovolts, the current I_(Y) provided by dependent current source 530 shouldbalance (or be equal) to the current I_(ANT) flowing through R_(ANT)515. The current I_(ANT) is given by:

$\begin{matrix}{I_{ANT} = {\frac{V_{Y} - V_{X}}{R_{ANT}} = {\frac{0 - V_{X}}{R_{ANT}} = \frac{- V_{X}}{R_{ANT}}}}} & (7)\end{matrix}$

To verify that the current I_(Y) provided by dependent current source530 is equal to the current I_(ANT), equation (7) can be set equal toequation (4) above. If the resulting equation holds true (or is valid)then V_(Y) is 0 volts and isolation has in fact been achieved:

$\begin{matrix}{I_{ANT} = {\frac{- V_{X}}{R_{ANT}} = {{V_{X} \cdot \left( {- g_{m\; 2}} \right)} = I_{Y}}}} & (8)\end{matrix}$

Because g_(m2) by design is equal to 1/R_(ANT), the above equation holdstrue so that the input of LNA 225 is isolated from the output of PA 220during reception.

3. CONCLUSION

Embodiments have been described above with the aid of functionalbuilding blocks illustrating the implementation of specified functionsand relationships thereof. The boundaries of these functional buildingblocks have been arbitrarily defined herein for the convenience of thedescription. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

1. A radio frequency (RF) front-end configured to support full-duplexcommunication, comprising: a gyrator comprising a first port coupled toan input of a low-noise amplifier (LNA) and a second port coupled to anoutput of a power amplifier (PA); and a transformer comprising a primarywinding coupled to an antenna and a secondary winding coupled betweenthe first port and the second port, wherein the gyrator is configured toisolate the output of the PA from the input of the LNA.
 2. The RFfront-end of claim 1, wherein the primary winding and the secondarywinding are inductively coupled.
 3. The RF front-end of claim 1, whereinthe gyrator further comprises: a first transistor comprising a sourcecoupled to ground, a drain coupled to the first port, and a gate coupledto the second port; and a second transistor comprising a source coupledto ground, a drain coupled to the second port, and a gate coupled to thefirst port.
 4. The RF front-end of claim 3, wherein the gyrator furthercomprises an inverting amplifier coupled between the first port and thegate of the second transistor.
 5. The RF front-end of claim 4, wherein:an input impedance of the first port is substantially equal to areciprocal of a transconductance of the first transistor; and an inputimpedance of the second port is substantially equal to a reciprocal of anegative transconductance of the second transistor.
 6. The RF front-endof claim 3, wherein the first transistor and the second transistor aremetal oxide semiconductor field effect transistors (MOSFETs).
 7. The RFfront-end of claim 6, wherein the first transistor and the secondtransistor are N-channel type MOSFETs.
 8. The RF front-end of claim 3,wherein a transconductance of the first transistor is substantiallyequal to a reciprocal of an equivalent resistance of the antenna as seenfrom the secondary winding.
 9. The RF front-end of claim 3, wherein atransconductance of the second transistor is substantially equal to areciprocal of an equivalent resistance of the antenna as seen from thesecondary winding.
 10. A radio frequency (RF) front-end configured tosupport full-duplex communication, comprising: a gyrator comprising: afirst transistor comprising a source coupled to ground, a drain coupledto a first port, and a gate coupled to a second port; and a secondtransistor comprising a source coupled to ground, a drain coupled to thesecond port, and a gate coupled to the first port; and a transformercomprising a primary winding coupled to an antenna and a secondarywinding coupled between the first port and the second port of thegyrator, wherein the gyrator is configured to isolate an input of a lownoise amplifier (LNA), coupled to the first port, from the output of apower amplifier (PA), coupled to the second port.
 11. The RF front-endof claim 10, wherein the primary winding and secondary winding areinductively coupled.
 12. The RF front-end of claim 10, wherein thegyrator further comprises an inverting amplifier coupled between thefirst port and the gate of the second transistor.
 13. The RF front-endof claim 12, wherein: an input impedance of the first port issubstantially equal to a reciprocal of a transconductance of the firsttransistor; and an input impedance of the second port is substantiallyequal to a reciprocal of a negative transconductance of the secondtransistor.
 14. The RF front-end of claim 10, wherein the firsttransistor and the second transistor are metal oxide semiconductor fieldeffect transistors (MOSFETs).
 15. The RF front-end of claim 14, whereinthe first transistor and the second transistor are N-channel typeMOSFETs.
 16. The RF front-end of claim 10, wherein a transconductance ofthe first transistor is substantially equal to a reciprocal of anequivalent resistance of the antenna as seen from the secondary winding.17. The RF front-end of claim 10, wherein a transconductance of thesecond transistor is substantially equal to a reciprocal of anequivalent resistance of the antenna as seen from the secondary winding.18. A radio frequency (RF) front-end configured to support full-duplexcommunication, comprising: a first transistor comprising a sourcecoupled to ground, a drain coupled to a first port, and a gate coupledto a second port; and a second transistor comprising a source coupled toground, a drain coupled to the second port, and a gate coupled to thefirst port, wherein the first and second transistor are configured toisolate an input of a low noise amplifier (LNA), coupled to the firstport, from the output of a power amplifier (PA), coupled to the secondport.
 19. The RF front-end of claim 18, further comprising an antennacoupled between the first port and the second port.
 20. The RF front-endof claim 18, further comprising an inverting amplifier coupled betweenthe first port and the gate of the second transistor.
 21. A radiofrequency (RF) front-end configured to support full-duplexcommunication, comprising: a gyrator comprising a first port coupled toan input of a receiver and a second port coupled to an output of atransmitter; and a transformer comprising a primary winding coupled to asignal transducer and a secondary winding coupled between the first portand the second port, wherein the gyrator is configured to isolate theoutput of the transmitter from the input of the receiver.